1. Field of Invention
The invention relates to a hybrid vehicle and a control method thereof.
2. Description of Related Art
A conventional hybrid vehicle in which a torque of the engine, that is, a portion of the torque is distributed to a generator motor and the remainder torque is distributed to a drive wheel, has a planetary gear unit that includes a sun gear, a ring gear and a carrier. The carrier is connected to the engine, the ring gear is connected to the drive wheel and the sun gear is connected to the generator motor. Rotation outputted from the ring gear and a drive motor is transferred to the drive wheel to produce a drive force.
In the hybrid vehicle constructed as described above, electric power is generated by the generator motor when the vehicle is running by driving the drive motor and the engine. If the rotation speed of the generator motor (the generator motor rotation speed) is low, the electric power consumption becomes great and the electric power generation efficiency correspondingly reduces. Therefore, a design is made as follows. That is, a brake as a fixing device is disposed between a rotor of the generator motor and a casing. If the generator motor rotation speed becomes equal to or less than 500 rpm, the brake is engaged to fix the rotation of the generator motor. After that, the generator motor rotation speed is calculated based on a rotation speed of an output shaft (an output rotation speed), of the planetary gear unit. The output rotation speed is calculated based on the vehicle speed and a revolution speed of the engine (an engine revolution speed) at an engine operation point where the power generation efficiency is high. If the generator motor rotation speed becomes greater than 500 rpm, the brake is released to remove the fixation of rotation to allow the rotor to rotate.
While the brake is in an engaged state, the torque of the engine, that is, the engine torque, acts on the rotor as a reaction force. Therefore, if the brake is merely released, the engine torque is transferred to the rotor so that the generator motor torque and the engine torque greatly change, thus causing a shock.
Therefore, the engine torque transferred to the rotor is estimated, and the brake is released after a generator motor torque corresponding to the estimated engine torque is produced in a direction opposite to the direction of the engine torque, to reduce the aforementioned occurrence of a shock. (Japanese Patent Application Laid-Open No. HEI 9-100853).
However, in the above-described conventional hybrid vehicle, a difference occurs between the estimated engine torque and the actual engine torque depending on the service condition of the engine, for example, the temperature of engine oil and the like, and furthermore, depending on individual variations of engines, for example, the inertia of the engine. Therefore, occurrence of a shock cannot be sufficiently suppressed.